1. FIELD OF THE INVENTION
The present invention relates to the deposition of epitaxial films on semiconductor substrates and, more particularly, to an improved process for masking the substrate surface to achieve selective patterning of the epitaxial film growth. Specifically, the present invention relates to the use of a bilayer masking scheme for the growth of epitaxial films such as gallium arsenide and germanium on semiconductor substrates such as silicon, using substrates that may already contain semiconductor devices.
2. DESCRIPTION OF THE PRIOR ART
The growth of high quality epitaxial films such as GaAs and/or Ge on dissimilar semiconductor substrate surfaces has many potential practical applications. However, any number of problems have arisen in attempting to grow such films. For example, attempts to grow GaAs directly on Si substrates have been limited by difficulties in nucleation and lattice mismatch. Despite the inherent problems in this material system, high quality GaAs has been grown on Si substrates both with and without an intermediate layer.
U.S. Pat. No. 4,111,725 discloses a process for growing GaAs epitaxial films on GaAs substrates. The technique disclosed in this patent involves molecular beam epitaxy (MBE) deposition of the GaAs onto the substrate surface through the use of an amorphous SiO.sub.2 masking layer. An SiO.sub.2 layer is first formed on the substrate, and then apertures are formed in the SiO.sub.2 mask using standard photolithographic techniques to create the appropriate patterns for GaAs growth. Once the expitaxial film is deposited through the patterned apertures, the amorphous layer is dissolved. This allegedly then "lifts off" any residual GaAs formed on top of the SiO.sub.2 mask.
A distinct problem with the process described in U.S. Pat. No. 4,111,725 arises when applying it to the growth of relatively thick epitaxial films on Si substrates or the like. Such thick epitaxial growth is highly desirable in lattice mismatched material systems in order to maximize the semiconductor properties thereof. Unfortunately, complete dissolution of the single amorphous masking layer so as to achieve a complete and clean lift-off from the substrate is very difficult using prior art techniques, particularly when the epitaxial film is thick.
A highly desirable application of this type of technology would include the formation of epitaxial layers such as GaAs/Ge on a Si substrate wherein the substrate already has Si semiconductor devices previously formed therein. However, without appropriate masking techniques, the underlying Si devices can be poisoned by the diffusion of Ga or As at the time of GaAs film deposition. Moreover, incomplete lift-off resulting from improper or incomplete masking techniques would interfere with subsequent metallization and processing, thus impeding the proper function of the resultant device.